Fix some array function parsing and serialization issues

Quite similar to #256 and also uses its test, but also fixes the serializer and doesn't try to parse an untyped 'x => x'.

tests/parser/arrow-functions.ts

@@ -0,0 +1,10 @@
+// array function
+(x): i32 => x;
+(x: i32) => x;
+(x?) => x;
+(x?, y?) => x;
+(x?: i32) => x;
+
+// not an array function
+(b ? x : y);
+(b ? f : g)();